The relation between ideas and images, in their different philosophical formulations since the Enlightenment, is one of the key elements of their definitions both in Episte-mology as well as in Aesthetics. This paper concentrates on Hume s theory of abstract ideas, based on his conception of ideas as images. I investigate first the roots of his theory in the works of Berkeley, to analyse further Hume s contention that his theory is completely nominalist. Here the idea of similarity, essential for the definition of a visual image, will show itself as the core of his theory.